Linear actuator

ABSTRACT

A linear actuator comprising a spindle, a spindle nut, a transmission, an electrical motor, and an actuation element, is arranged to linearly move the actuation element by means of an interaction of the spindle and the spindle nut, which interaction is being driven by the electrical motor through the transmission. A position of the actuation element, the relative position between spindle and spindle nut, is determined by means of an absolute rotary position sensor and a counter. The counter keeps track of the number of under-flows and over-flows the absolute rotary position sensor generates during movement of the actuation element. A combination of a value from the absolute rotary position sensor and a count from the counter determines the position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/813,239, filed Jan. 30, 2013, which is a U.S. National Stage ofInternational Application No. PCT/EP2010/004666 filed on Jul. 30, 2010.

TECHNICAL FIELD

The invention is related to linear actuators, especially to linearactuators driven by an electrical motor, in particular the determinationof the position of an actuation element of a linear actuator.

BACKGROUND

Linear actuators are commonly used when a controlled linear motion isdesired instead of a rotational movement. One use of linear actuators isto be able to remotely open and close windows, for example ventilationwindows of green houses. Pneumatic and hydraulic actuators have beenused as they are commonly linear by nature. Both pneumatic and hydraulicactuators have many disadvantages, energy efficiency being one, which iswhy there has for some time been a shift towards linear actuators drivenby electrical motors.

To determine the position of an actuation element of a linear actuator,linear actuators are commonly provided with position sensors. Formerlyrotary potentiometers were used. Due to the mechanical nature ofpotentiometers, they wear out, thus lowering the reliability of thelinear actuators they were used in. More modern linear actuators willmost likely use wear resistant technology such as semiconductorHall-sensors that are contact-less sensors. The US patent application US2009/0091287 describes a linear actuator making use of Hall-sensors.Unfortunately the suggested system requires that the sensors and microprocessor always need to be active. Thus there seems to be room forimprovement in determining a position of an actuation element of alinear actuator.

SUMMARY

An object of the invention is to define a method and means to determinethe position of an actuation element of a linear actuator.

The aforementioned object is achieved according to the invention by theuse of a linear actuator comprising a spindle, a spindle nut, atransmission, an electrical motor, and an actuation element, which isarranged to linearly move the actuation element by means of aninteraction of the spindle and the spindle nut, which interaction isbeing driven by the electrical motor through the transmission. Theactuation element is moved between a first end stop and a second endstop, defining a complete stroke. A position of the actuation element,the relative position between spindle and spindle nut, is determined bymeans of an absolute rotary position sensor and a counter. The absoluterotary position sensor is coupled to a rotating part of the linearactuator and will under-flow/over-flow at least once during a completestroke; that is the rotational angle through which the absoluterotational position sensor can determine the angle of, is less than thecomplete rotational angle of the rotating part during a complete stroke.Thus during a complete stroke, the absolute rotary position sensor willunder-flow, that is go from an output value representing the smallestangle to an output value representing the largest angle, or over-flow,that is go from an output value representing the largest angle to anoutput value representing the smallest angle, at least once. The counterkeeps track of the number of under-flows and over-flows the absoluterotary position sensor generates during movement of the actuationelement. A combination of a value from the absolute rotary positionsensor and a count from the counter determines the position.

The aforementioned object is further achieved according to the inventionby a linear actuator comprising an electrical motor, a transmission, aspindle, a spindle nut, an actuation element and a control unit. Theelectrical motor, through the transmission and interaction betweenspindle and spindle nut, drives the actuation element to move linearlybetween a first end stop and a second end stop. According to theinvention, to determine a position of the actuation element, the controlunit comprises an absolute rotary position sensor outputting a valuerepresenting an angle. The value can be the angle or just a numericvalue that represents an angle, for example a 10 or 12 bit number. Thecontrol unit further comprises a counter keeping a count. The absoluterotary position sensor is coupled to the linear actuator in such a waythat the rotational angle through which the absolute rotational positionsensor will give an absolute value is less than the rotational anglethrough which the absolute rotational position sensor is subjectedduring a movement of the actuation element between the first end stopand the second end stop. This causes the value of the absoluterotational position sensor to under-flow or over-flow at least onceduring such a movement between end stops. The counter is arranged tokeep track of the under-flows and over-flows of the value in the count.Thus the combination of the value of the absolute rotational positionsensor and the count of the counter, determines the position of theactuation element. The absolute rotary position sensor can be a fullturn or a partial turn absolute rotary position sensor. The control unitfurther comprises a non-volatile memory. The control unit is thenarranged to store the count of the counter in the non-volatile memorywhen there is a shutdown of the linear actuator or, as a way of ensuringthat track is kept of the count, when the rate of movement of theactuator element as indicated by the absolute rotary position sensor,falls below a predetermined limit. The shutdown can be by command or byremoval of power from the linear actuator.

The control unit can suitably in some embodiments also be arranged whenthe count of the counter is stored when there is a shutdown to alsostore the count of the counter in the non-volatile memory when the rateof movement of the actuation element as indicated by the absolute rotaryposition sensor, falls below a predetermined limit. This in case theinertia of the system keeps the actuation element moving after ashutdown/removal of power from the electrical motor.

If the embodiment comprises a non-volatile memory, then in addition tostoring the count, the control unit can be arranged to simultaneouslystore in the non-volatile memory the value of the absolute rotaryposition sensor. This can then in some embodiments allow the controlunit to be arranged to perform a check during power-up of the linearactuator to determine if there is a difference between a stored value ofthe absolute rotary position sensor in the non-volatile memory and avalue of the absolute rotary position sensor at power-up, and if thereis a difference to then determine if that difference is withinpredetermined limits or not. Has the actuation element of the linearactuator moved during a shut down, and if so by how much. The detectablemovement is restricted by the range of the absolute rotary positionsensor since the counter is not updated during shutdown/when power isremoved.

The control unit can in some embodiments either comprise a capacitor ora battery to provide energy for storage of the count of the counter andpossibly the value of the absolute rotary position sensor to thenon-volatile memory. The additional energy is to provide the possibilityto at least store the count of the counter and possibly also to measureand store the value of the absolute rotary position sensor even if allexternal power is removed.

Suitably the control unit is also arranged to determine end stops of thelinear actuator. This is done by driving the electrical motor in bothdirections until the absolute rotary position sensor indicates that theactuation element is not moving. The count of the counter and the valueof the absolute rotary position sensor are used. Suitably the controlunit is arranged to drive the electrical motor at a limited torque/powerlevel during end-stop determination. The control unit is preferablyarranged to determine the end stops to be at a predetermined distancebefore the position indicated by the absolute rotary position sensor.When the absolute rotary position sensor indicates that an end stop hasbeen reached by the absence of change of the value, then the motor isreversed until the absolute rotary position sensor starts to output achanging value that indicates that the actuation element has started tomove. This position can be used as end stop, or a position pulled backeven further back a predetermined amount.

The control unit can be arranged, by means of the absolute rotaryposition sensor, to determine if there is movement of the actuationelement when the actuator is not driven by the electrical motor. If itis determined that there is movement even though the actuator is notdriven, then an alarm message can be generated and communicated and/orthe electrical motor can be activated to oppose this movement and thuscreate an active stop of movement.

By using an absolute rotary position sensor in combination with storinga count in non-volatile memory, a linear actuator according to theinvention can be calibrated and checked and then stored/be powerless fora virtually infinite amount of time since it does not have to rely onbatteries or another power source to continuously keep sensors and amicro processor active to not loose calibration or other settings.

Other advantages of the invention will become apparent from the detaileddescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory, andin no sense limiting, purposes, with reference to the following figures,in which:

FIG. 1 shows a view of a linear actuator unit according to theinvention,

FIG. 2 shows a cross section of a part of the linear actuator unitaccording to the invention,

FIG. 3 shows a functional block diagram of the controller and absoluteposition sensor according to the invention,

FIGS. 4A-4C show different types of absolute rotary position sensors andtheir output over a full turn,

FIG. 5 shows a flow chart of a basic functionality of the absoluteposition sensor according to the invention,

FIG. 6 shows a flow chart of an optional check of a linear actuator withan absolute position sensor according to the invention,

FIG. 7 shows a flow chart of an optional virtual end stop determinationof a linear actuator with an absolute position sensor according to theinvention.

DETAILED DESCRIPTION

In order to clarify the inventions, some examples of its use will now bedescribed in connection with FIGS. 1 to 7

FIG. 1 shows a view of a linear actuator unit 100 according to theinvention. The linear actuator 100 comprises an electrical motor 114which is coupled to a transmission 112, which in turn is coupled to aspindle and spindle nut arrangement to thereby linearly drive anactuation rod/element 111. The spindle and spindle nut arrangement, inthis illustration covered by the spindle outer tube 110, will convert arotary motion of the electrical motor 114 and transmission 112 to alinear motion of the actuation rod/element 111. Either the spindle orthe spindle nut is driven by/coupled to the transmission depending onthe type of linear actuator.

The linear actuator 100 according to the invention further comprises anabsolute rotary position sensor 150, control electronics 116 and apower/control connector 118. The power/control connector(s) 118 are toprovide the linear actuator 100 with power and a control connection toeither discrete control switches, a bus system, serial or parallel,according to an industry standard or a proprietary one, or a connectionto a network, industry standard or proprietary, for controlling andpossibly also monitoring the linear actuator. In some embodiments, acontrol connection is not needed as these linear actuators are directlycontrolled by connection and reversing of the power.

In some embodiments the control electronics 116 will comprise circuitryfor conversion of the value output of the absolute rotary positionsensor, into a linear position of the actuation rod/element 111. Oftenit is not necessary to know a numerical value of the position, but onlyfor example when an end stop is being reached.

FIG. 2 shows a part of the linear actuator unit according to theinvention with a cover of the control electronics removed thusdisplaying a circuit board 217 comprising circuitry 256 to determine therotary position of an absolute rotary position sensor 252, 254. Theabsolute rotary position sensor 252, 254 will suitably be of a full turnor partial turn type and preferably not of a multi-turn type as theseare more expensive and not necessary according to the invention. As therotating part to which the absolute rotary position sensor 252, 254 iscoupled 213 will be of a multi-turn nature, then the circuitry 256further comprises a counter to count the number of over/under flows ofthe absolute rotary position sensor 252, 254. The absolute rotaryposition sensor 252, 254 can be mechanically coupled 213 to a rotatingpart such as a rotor of the electrical motor, an axle or gear of thetransmission 212, or directly coupled 213 to the spindle or spindle nutof the linear actuator. It is preferable to couple the absolute rotaryposition sensor to the spindle/screw instead of the motor or anotherpart of the transmission as the measurement then does not get subjectedto any play in the gear train. It even makes it possible to correct forgear train play.

The absolute rotary position sensor 252, 254 suitably comprises twoparts, one rotating 252 and one stationary. The rotating part 252 of theabsolute rotational position sensor, is suitably a magnet or magnetring, mounted directly or indirectly, i.e. coupled to, a rotating partof the linear actuator, such as a spindle or spindle nut. Thenon-rotating part 254 of the absolute position sensor, suitably a halleffect sensor, mounted in proximity to the rotating part 252 of theabsolute position sensor, suitably on or coupled to the circuit board.

The circuitry 256 suitably also comprises non-volatile memory forstorage of the count at the time of power shutdown. To properly enablethe function of saving the count and optionally the value of theabsolute rotational position sensor at or during shutdown, there areembodiments comprising a power storage unit 258, such as acapacitor/battery to ensure that there is enough power to store thecount and possibly the value, when the linear actuator is shut down.

Finally there is provided means to pass the electrical power 219 throughto the electrical motor.

FIG. 3 illustrates a functional block diagram of the controller 360,absolute rotary position sensor 350 and surrounding circuitry accordingto the invention. There will be electrical power supply line(s) 340 thatare regulated and controlled 348 before being fed as power 349 to anelectrical motor (not shown). Control signals 364 from the controller360 determine how the control circuitry 348 for the electrical motorwill regulate the power supply 340. The electrical motor can becontrolled to determine end stops, be given a lower power close to theend stops, determine direction of operation and also be provided withsafety functions such as overload protection. End stop determination andoverload protection are according to the invention done in conjunctionwith the absolute rotary position sensor 350. If a rate of movementaccording to the absolute rotary position sensor 350 changes for a givenamount of power, this could be an indication of approaching an end stopduring end stop calibration; or, during ordinary operation if the linearactuator is not close to an end stop, could be an indication ofincreasing load. It is also possible to detect any drift in themechanical system when the actuator is not driven. If movement isdetected, either a warning/error message can be generated and sentand/or the motor can be activated to counter this undesired movement,i.e. act as an active stop.

The electronic circuitry is supplied with regulated power through apower supply regulator 343. The regulator 343 suitably has reserve power358 comprising for example a capacitor or a battery. The reserve power358 is used to ensure that the controller 360, non-volatile memory 370and absolute rotary position sensor 350 have sufficient power to fulfillshutdown sequences even though the main power 340 has gone. Shutdownsequences will typically comprise saving a shutdown count and possibly ashutdown value into the non-volatile memory 370.

Preferably the controller 360, such as a micro controller or ASIC, isconnected to a control bus or network through one or more control lines362. Such a control network can then suitably control more than onelinear actuator.

FIGS. 4A, 4B, and 4C show different types of absolute rotary positionsensors and their output over a full turn of a rotating element that theabsolute rotary position sensor is connected to. The figures illustratethe absolute rotary position sensor output 405 in relation to rotationalangle 407, where one full turn 409 is indicated. FIG. 4A illustrates anabsolute rotary position sensor with a full turn 470 unambiguous output.FIG. 4B illustrates an absolute rotary position sensor with a third of afull turn 473, 474, 475 of unambiguous output. And finally FIG. 4Cillustrates an absolute rotary position sensor with less than a fullturn 478, 479 of unambiguous output. Since the linear actuator accordingto the invention also uses an under-flow/over-flow count, it does notmatter if the absolute rotary position sensor has a full turn or apartial turn unambiguous output. In FIG. 4B, there is illustrated anoverflow 481 when the output value 405 goes from a maximum anglerepresentation to a minimum angle representation and also illustrated anunderflow 483 when the output value goes from a minimum anglerepresentation to a maximum angle representation.

It would be ideal to have a multi-turn absolute rotary position sensorwith more turns than a measured rotating member will rotate during afull stroke of the actuation element. However, with the currentinvention of having a limited absolute rotary position sensor incombination with an over-/under-flow counter, a cheap and robustsolution is given. The linear actuator according to the invention isable to determine if there has been movement of the actuation elementduring shutdown. The value of the absolute rotary position sensor has tobe saved and the allowable/detectable movement during shutdown islimited by the range of the absolute rotary position sensor. Detectable,thus allowable movement during shutdown is plus/minus half the range ofthe absolute rotary position sensor. The solution according to theinvention does not rely on any power being supplied when the linearactuator is shut off. It is only required during a shutdown phase to beable to get a higher accuracy of where the actuation element really iswhen stopped.

FIG. 5 shows a flow chart of a basic functionality of the absoluteposition sensor according to the invention. In a first step 510 it isdetermined if there is a rotation or not. In a second step 520, if thereis rotation, it is determined if we are still within the absoluterotational position sensor range, or if there is an under or overflow ofthe sensor output. In a third step 530, if it is determined that thereis an under- or over-flow, then the counter is adjusted accordingly bycounting up one if there has been an overflow and by counting down oneif there has been an underflow. In a fourth step 540 it is determined ifthere is a power loss/shutdown or not. In a fifth step 550 if it isdetermined that there is a power loss/shutdown, then the count is storedin a non-volatile memory. Optionally in a sixth step 560, if there is apower loss/shutdown, then the absolute rotary position sensor value isalso stored in a non-volatile memory.

FIG. 6 shows a flow chart of an optional check of a linear actuator withan absolute position sensor according to the invention. In a first step600 it is determined if there is a power-up or not, if there is, thenthis routine/process is executed. In a second step 610 a stored count inthe non-volatile memory is retrieved and the counter is updated withthis count. In a third step 620 a stored value of the absolute rotaryposition sensor in the non-volatile memory is retrieved. In a fourthstep 630 a current value of the absolute rotary position sensor isacquired/determined. In a fifth step 640 the retrieved stored value iscompared with the determined current value of the absolute rotaryposition sensor, to determine if there has been an underflow/overflowduring shutdown or not. In a sixth step 650, if it determined that therehas been an overflow/underflow, then the counter is updated accordinglyby counting up one if there has been an overflow and by counting downone if there has been an underflow. In a seventh step 660 the retrievedstored value is compared with the determined current value, and if thedifference is less than a first predetermined amount, then theroutine/process is terminated without further action with an eleventhstep 699. If the difference is equal or greater than the firstpredetermined amount, then the process continues with an eighth step 670where the retrieved stored value is compared with the determined currentvalue in relation to a second predetermined amount. If it is determinedthat the difference is less than the second predetermined amount then ina ninth step 680 a warning action is performed. If it is determined thatthe difference is equal or greater than the second predetermined amountthen in a tenth step 690 an error action is performed. The error actioncan include a recalibration. In the eleventh step 699 further processingafter power-up is done.

FIG. 7 shows a flow chart of an optional virtual end stop determinationof a linear actuator with an absolute position sensor according to theinvention. In a first step 705 it is determined if it is the firstpower-up or a recalibration, if so then this routine/process isexecuted. In a second step 710 the electrical motor power is set to alow/calibration value. In a third step 715 the electrical motor/linearactuator is set to run in a first direction. In a fourth step 720 it isdetermined if the absolute rotary position sensor registers a movementor not. In a fifth step 725, when it is determined that the movement hasstopped, then the electrical motor is stopped. In a sixth step 730 acount of the counter and a value of the absolute rotary position sensoris determined In a seventh step 735 the first end stop is set to theposition determined by the absolute rotary position sensor value and thecount of the counter minus a predetermined amount. In an eighth step 740the electrical motor/linear actuator is set to run in a seconddirection. In a ninth step 745 it is determined if the absolute rotaryposition sensor registers a movement or not. In a tenth step 750, if itis determined that the movement has stopped, then the electrical motoris stopped. In an eleventh step 755 the count of the counter is read andthe value of the absolute rotary position sensor is read. In a twelfthstep 760 a second end stop is set to the position determined by theabsolute rotary position sensor and counter, minus a predeterminedamount. In a thirteenth step 765 the electrical motor power is set to anormal/nominal value. The fourteenth step 770 exits the routine/process.

The invention is not restricted to the above-described embodiments, butmay be varied within the scope of the following claims.

FIG. 1 shows a view of a linear actuator unit according to theinvention,

-   100 A linear actuator unit according to the invention-   110 Spindle outer tube-   111 Actuation rod, inner tube,-   112 Transmission,-   114 Electrical motor,-   116 Control electronics,-   118 Connector, power/control,-   150 Absolute rotary position sensor.

FIG. 2 shows a cross section of a part of the linear actuator unitaccording to the invention,

-   212 Transmission,-   213 Coupling to absolute position sensor,-   217 Circuit board,-   218 Power/control connector,-   219 Connector to electrical motor,-   252 First part of absolute position sensor, suitably a magnet or    magnet ring, mounted directly or indirectly, i.e. coupled to, a    rotating part of the linear actuator, such as a spindle or nut-   254 Second part of absolute position sensor, suitably a hall effect    sensor, mounted in proximity to the first part of the absolute    position sensor, suitably on or coupled to the circuit board,-   256 Circuitry to determine the rotary position, comprising a counter    to count the number of over/under flows of the absolute rotary    position sensor, and a memory for storage of the count at the time    of power shutdown,-   258 Power storage, such as a capacitor/battery to ensure that there    is enough power to store the count when the linear actuator is shut    down.

FIG. 3 shows a functional block diagram of the controller and absoluteposition sensor according to the invention,

-   340 Electrical power supply line(s),-   343 Power supply regulator for electronic circuitry,-   345 Regulated power supply to electronic circuitry,-   348 Control circuitry for electrical motor power,-   349 Power to electrical motor,-   350 Absolute rotary position sensor,-   358 Electrical power storage, such as a capacitor or battery, for    supplying power to electronic circuitry during linear actuator    shutdown,-   360 Controller, such as a micro controller or ASIC (Application    Specific Integrated Circuit),-   362 Optional control signal lines,-   364 Control signals from controller to control circuitry for    electrical motor,-   370 Memory for storage during shut down of a count of over/under    flow of absolute rotary position sensor.

FIGS. 4A, 4B, and 4C show different types of absolute rotary positionsensors and their output over a full turn.

-   405 Absolute rotary position sensor output,-   407 Rotational position of spindle/nut/motor rotor/gear in    transmission,-   409 One full turn of spindle/nut/motor rotor/gear in transmission to    which absolute rotary position sensor is coupled,-   470 One full turn output of a one full turn absolute rotary position    sensor,-   473 First full range output of a one third partial turn absolute    rotary position sensor,-   474 Second full range output of a one third partial turn absolute    rotary position sensor,-   475 Third full range output of a one third partial turn absolute    rotary position sensor,-   478 First full range output of a partial turn absolute rotary    position sensor,-   479 Start of second full range output of a partial turn absolute    rotary position sensor

FIG. 5 shows a flow chart of a basic functionality of the absoluteposition sensor according to the invention,

-   510 Is there rotation?,-   520 If there is rotation, are we still within absolute rotational    position sensor range, or is there an under or overflow,-   530 If there is an under or overflow, then adjust counter    accordingly,-   540 Is there a power loss/shutdown?-   550 If there is a power loss/shutdown, then store count in    non-volatile memory,-   560 Optionally, if there is a power loss/shutdown, then also store    the absolute rotary position sensor value.

FIG. 6 shows a flow chart of an optional check of a linear actuator withan absolute position sensor according to the invention,

-   600 If there is a power-up then do this routine/process,-   610 Retrieve count stored in the non-volatile memory and update    counter with this count,-   620 Retrieve absolute rotary position sensor value stored in the    non-volatile memory,-   630 Acquire/determine current value of absolute rotary position    sensor,-   640 Compare retrieved value with current value, and if the    difference shows that there has been an under or overflow during    shutdown, then continue with step 650, otherwise continue with step    660,-   650 If there has been determined that there has been an overflow or    underflow during shutdown, then adjust counter accordingly,-   660 Compare retrieved value with current value, and if the    difference is less than a first amount, then exit routine/process    without further action, otherwise continue with step 670-   670 Compare retrieved value with current value in relation to a    second amount,-   680 If the difference is less than the second amount then do a    warning action, can be a message sent back to a controller via    communication network,-   690 If the difference is equal or greater than the second amount    then do an error action, such as a recalibration,-   699 Further processing after power-up.

FIG. 7 shows a flow chart of an optional virtual end stop determinationof a linear actuator with an absolute position sensor according to theinvention,

-   705 If it is the first power-up or a recalibration, then do this    routine/process,-   710 Set electrical motor power to low/calibration value,-   715 Set electrical motor/linear actuator to run in a first    direction,-   720 Check that the absolute rotary position sensor registers a    movement,-   725 When movement has stopped, then stop electrical motor,-   730 Read count of counter, read value of absolute rotary position    sensor,-   735 Set first end stop to position determined by absolute rotary    position sensor and counter minus a predetermined value,-   740 Set electrical motor/linear actuator to run in a second    direction,-   745 Check that the absolute rotary position sensor registers a    movement,-   750 When movement has stopped, then stop electrical motor,-   755 Read count of counter, read value of absolute rotary position    sensor,-   760 Set second end stop to position determined by absolute rotary    position sensor and counter minus a predetermined value,-   765 Set electrical motor power to normal value,-   770 Exit routine/process.

1. A linear actuator comprising an electrical motor, a transmission, aspindle, a spindle nut, an actuation element and a control unit, wherethe electrical motor through the transmission and interaction betweenspindle and spindle nut drives the actuation element to move linearlybetween a first end stop and a second end stop, characterized in that todetermine a position of the actuation element the control unit comprisesan absolute rotary position sensor outputting a value representing anangle and comprises a counter keeping a count, where the absolute rotaryposition sensor is coupled to the linear actuator in such a way that therotational angle through which the absolute rotational position sensorwill give an absolute value is less than the rotational angle throughwhich the absolute rotational position sensor is subjected to during amovement of the actuation element between the first end stop and thesecond end stop, causing the value of the absolute rotational positionsensor to under-flow or over-flow at least once during such a movementand where the counter is arranged to keep track of the under-flows andover-flows of the value in the count, characterized in that the controlunit further comprises a non-volatile memory and in that the controlunit is arranged to store the count of the counter in the non-volatilememory when there is a shutdown of the linear actuator or when the rateof movement of the actuator element as indicated by the absolute rotaryposition sensor, falls below a predetermined limit.
 2. The linearactuator according to claim 1, further comprising if the count of thecounter is stored in the non-volatile memory when the there is ashutdown of the linear actuator, then the control unit is furtherarranged to also store the count of the counter in non-volatile memorywhen the rate of movement of the actuator element as indicated by theabsolute rotary position sensor, falls below a predetermined limit. 3.The linear actuator according to claim 1, wherein the control unit isarranged to simultaneously store in the non-volatile memory the value ofthe absolute rotary position sensor.
 4. The linear actuator according toclaim 3, wherein the control unit is arranged to perform a check duringpower-up of the linear actuator to determine if there is a differencebetween a stored value of the absolute rotary position sensor in thenon-volatile memory and a value of the absolute rotary position sensorat power-up, and if there is a difference to then determine if thatdifference is within predetermined limits or not.
 5. The linear actuatoraccording to claim 1, wherein the control unit further comprises acapacitor to provide energy for storage of the count of the counter tothe non-volatile memory.
 6. The linear actuator according to claim 1,wherein the control unit further comprises a battery to provide energyfor storage of the count of the counter to the non-volatile memory. 7.The linear actuator according to claim 1, wherein the control unit isarranged to determine end stops of the linear actuator by driving theelectrical motor in both directions until the absolute rotary positionsensor indicates that the actuation element is not moving.
 8. The linearactuator according to claim 7, wherein the control unit is arranged todrive the electrical motor at a limited torque/power level duringend-stop determination.
 9. The linear actuator according to claim 7,wherein the control unit is arranged to determine the end stops to be ata predetermined distance before the position indicated by the absoluterotary position sensor.
 10. The linear actuator according to claim 1,wherein the control unit is arranged by means of the absolute rotaryposition sensor to determine if there is movement of the actuationelement when the actuator is not driven by the electrical motor, and ifthis is determined, then an alarm message is generated.
 11. The linearactuator according to claim 1, wherein the control unit is arranged bymeans of the absolute rotary position sensor to determine if there ismovement of the actuation element when the actuator is not driven by theelectrical motor, and if this is determined, then the electrical motoris activated to oppose this movement and thus create an active stop ofmovement.